Well, I think, almost everything that we have here on forums can be found in Wikipedia or elsewhere, on similar resources. I think, it will still be of use if we write the names of the characters here. There are not so many of them after all.

In Turkey, we usually do not change the names of the characters, they remain the same as in their original languages. Only names like "Rabbit" and "Owl" may be translated since these names already exist in Turkish. Then it would be Rabbit: Tavşan Owl: Baykuş

I think, it rather refers to an imaginary animal that parents might talk of when they want to frighten their little children: «веди себя прилично, не то придёт бука!" — "behave decently, or a bookah will come!".